Co-pending patent application Ser. No. 582,305, filed Sep. 14, 1990, by S. Kristy, entitled "Multiresolution Digital Imagery Photofinishing System," assigned to the assignee of the present application and the disclosure of which is incorporated herein, discloses a system diagrammatically illustrated in FIG. 1, in which photographic images, such as a set of twenty-four or thirty-six 24 mm.times.36 mm image frames of a 35 mm film strip 10, are digitized, processed in accordance with prescribed image reproduction characteristics and then stored on a compact disc for subsequent playback, as by way of a consumer television display. At the front end of the process, the film strip is scanned by a high resolution opto-electronic film scanner 12, such as a commercially available Eikonix Model 1435 scanner. Scanner 12 outputs digitally encoded data (e.g. a 2048.times.3072 pixel matrix) representative of the internal electronic scanning of a high resolution image sensing array onto which a respective photographic image frame of film strip 10 is projected. This digitally encoded data, or `digitized` image, is coupled in the form of an imaging pixel array-representative bit map to an attendant image processing (photofinishing) workstation 14, which contains a frame store and image processing application software through which the digitized image is processed (e.g. enlarged, rotated, cropped, subjected to scene balance correction, etc.) to achieve a desired image appearance. Once an image file has been prepared, it is stored on a transportable medium, such as a write-once optical compact disc, using an optical compact disc recorder 16, for subsequent playback by a disc player 20, which allows the image to be displayed, for example, on a relatively moderate resolution consumer television set 22 (e.g. having an NTSC display containing an array of 485 lines by 640 pixels per line), or printed as a finished color print, using a high resolution thermal color printer 24.
One particular mechanism for controlling the manner in which a digitized image that has been recorded on disc is read out and displayed is described in copending U.S. patent application, Ser. No. 583,265, filed Sep. 14, 1990 by Parulski, et al, entitled "Mechanism for Controlling Presentation of Displayed Image", assigned to the assignee of the present application and the disclosure of which is incorporated herein. In accordance with this display mechanism, two dimensional image data of a prescribed spatial resolution less than that of the display device itself is subjected to a decimation or cropping mechanism in the course of reading out the imagery data.
Because of their very high resolution, the video images stored on disc contain more detail than can be displayed using a conventional TV display, reproduction signals for which typically originate with NTSC or PAL format video cameras. In particular, 35 mm color film images processed in accordance with the mechanisms described in the above referenced Kristy and Parulski et al applications have the capacity to store a much larger quantity of vertical high spatial frequency information than do conventional TV images derived from video cameras. This additional vertical high spatial frequency content sometimes causes a raster scan display artifact known as "interlace flicker" where the vertical edge details (associated with sharp horizontal lines, for example) will flicker visibly at a 30 Hz rate on normal interlaced NTSC displays. This interlace flicker occurs because the TV monitor displays every other line of the image during the first 1/60 second field time, and then displays the lines in between during the next 1/60 second field time, as diagrammatically shown in FIG. 2. In regions of an image having significant vertical detail, the lines of the first field will be quite different from the lines of the second field, so these regions of the image will appear to flicker at a 30 Hz rate. The amount of interlace flicker in an image will depend upon the image content and the way in which the image was photographed.
FIG. 3 is a simplified block diagram of the relevant portion of circuitry incorporated in a conventional, currently commercially available improved definition television (IDTV) system for reducing interlace flicker by improving vertical definition. In such an IDTV system, a receiver up-converts a 2:1 interlaced television signal to a progressive scan display format for use with a 32 KHz line rate CRT. In accordance with the operation of an IDTV system, rather than interlacing alternating raster lines, each line of the display is scanned every 1/60 second. Since conventional NTSC television signals provide video data for scanning every other raster line each 1/60 second, for each 1/60 second the IDTV system generates video data for the display lines which are not scanned in a conventional television system.
Since television systems normally display moving images, the image data to be scanned onto the interlaced raster lines must be generated while taking the motion into account. This is true even if the television source is a still image, since still images may be manipulated in such a way as to be temporarily in motion. Manipulation of still images typically includes zooming (i.e., magnifying) the still images being displayed, panning (i.e., moving side-to-side or up-and-down) through the image, and changing from one still image to a different still image.
An image data pixel value for a pixel of a raster line not conventionally scanned can be generated by an IDTV system by using a number of signal processing mechanisms. One common approach is to generate a median value for the pixel based on the current values of the image data pixels immediately above and below the pixel to be generated, and on the value of the pixel during the previous 1/60 second, when the raster line containing the pixel was scanned.
The IDTV system of FIG. 3 includes a memory, shown as a field store 101, for storing video data extracted from a video input signal received by the system for display. Also shown is a delay line 102, coupled to receive the input video signal. A median filter 104 is used to produce pixel data for the raster lines not provided by the interlaced video input. The median filter 104 produces median data, i.e., interpolated data, for a given raster line based on the current data being scanned on the upper and lower adjacent raster lines and data which was scanned onto the given raster line during the previous 1/60 second. The median filter 104 has inputs coupled to outputs of the field store 101 and the delay 102, and has another input coupled to directly receive the input image data. The input video data and the output of the median filter 104 are subjected to 2:1 time compression operations at 106A and 106B. The compressed signals are applied to respective inputs of a 2:1 multiplexer, the output of which is coupled to a display unit 108, shown as a 60 frame/sec progressive scan display.
An IDTV system employing a median filter produces satisfactory results with a moving image such as regular television programming, or with a changing still image such as-an image zooming or panning responsive to user commands. However, a median filter is not well suited for producing progressive scan conversion on stationary still images, such as those to be played back from a photo compact disc system, because the median filter acts to reduce the definition of images having high spatial frequencies in the vertical direction. In addition, in practice it has been found that such filtration actually adds flicker in still images because of the interlaced scanning.
To understand why this is the case, consider a situation in which first, second, third, and fourth raster lines of a stationary still image are alternating black, white, black, and white, respectively. During a first 1/60 second, the first and third lines are scanned with black data. The median filter generates interpolated data for the second line based on the present data for the first and third lines (both black) and the data for the second line from the previous 1/60 second (white). One white and two blacks produce a median value of black. Thus, the second line is scanned black. During the next 1/60 second, the second and fourth lines are scanned white. The median filter generates interpolated data for the third line based on the present data for the second and fourth lines (both white) and the data for the third line from the first 1/60 second (black). Now, two whites and one black produce a median value of white. Thus, the third line is scanned white. It will be seen, therefore, that a median filter both reduces definition and generates flicker, both undesirable results, if an interpolated median value for a stationary still image is different from the actual value because of differing adjacent values.
Reproduction signals produced by CD player 20 (FIG. 1) may be encoded into NTSC composite color signals or PAL composite color signals. To form composite color signals, red, green and blue component signals are subjected to a transform matrix to form a luminance signal (Y) and a pair of color difference signals (C1), (C2). The two color difference signals are modulated in quadrature on a high frequency color subcarrier. This color encoding process is described, for example, in the publication entitled "Television Engineering Handbook," Second Edition, edited by K. Blain Benson, McGraw Hill Book Company, New York 1986.
Because the luminance and modulated color difference signals are combined into a single NTSC or PAL composite signal, the NTSC or PAL television display incorporates circuitry to decode the composite color signal back into the red, green and blue component signals, thereby enabling the color image to be properly displayed. Two well known artifacts known as "cross color" and "cross luminance", and which are described in detail in an article entitled "Improving NTSC to Achieve Near-RGB Performance" by Yves Faroudja and Joseph Roizen, SMPTE Journal, August 1987, pp 750-761, are associated with images that have been decoded from NTSC or PAL composite color signals. The first of these artifacts, known as "cross color", occurs when high frequency luminance information is mistakenly decoded as color information. The second artifact, known as "cross luminance", occurs when chrominance information is mistakenly decoded as high frequency luminance information.
So long as the image contains no motion, from frame to frame, both cross luminance and cross color can be eliminated by using a frame delay comb filter to separate the luminance signal and the quadrature modulated chrominance signal. U.S. Patent to Achiha et al, U.S. Pat. No. 4,530,004, entitled "Color Television Signal Processing Circuit" describes circuitry, diagrammatically illustrated in FIG. 4, for separating luminance and chrominance signals on the basis of the difference signals from the composite color television signal between adjacent fields or frames. As detailed in the Achiha et al patent, an NTSC composite color television signal fed to a circuit input terminal 9 is applied to a subtraction circuit 11 along with a signal associated with the preceding frame and which is delayed by means of a frame memory 10 having a storage capacity of 525H by an amount corresponding to one frame period of the television signal, (where H represents the time for one horizontal scanning period). Subtraction circuit 11 produces an output signal in which the chrominance carrier component is doubled in magnitude and the luminance component is removed. Accordingly, when the output signal from subtraction circuit 11 is passed through a coefficient circuit 12 with a coefficient of one-half, a signal is produced which is an average of the chrominance component over the inter-frame. This average signal is then passed through a band-pass filter 13, thus producing at output terminal 17 a signal having a chrominance carrier signal C from which the luminance signal component has been removed. On the other hand, the NTSC signal is partly fed to a delay circuit 14 having the same delay time as bandpass filter 13 for delay time adjustment and is then applied to subtraction circuit 15, wherein the chrominance carrier signal C is subtracted from the NTSC signal, to produce the luminance component Y of the NTSC signal.
If the image contains motion, artifacts similar to cross-luminance and cross color may be produced by the circuit of FIG. 4, when moving high frequency luminance information is mistakenly decoded as color information, and moving high frequency chrominance information is mistakenly decoded as color information. As a result, simple frame rate comb filters cannot provide satisfactory decoding of composite color signals from moving television images.
An additional problem occurring in video images is the fact that transmitted television signals often contain significant amounts of noise that are injected into the television signal path. Such noise is especially noticeable when viewing still television images, since moving objects or rapidly changing scenes tend to visually mask the noise to some degree.
FIG. 5 diagrammatically illustrates an example of a recursive filter circuit that may be employed to reduce noise in television images. In particular, FIG. 5 illustrates a frame averaging circuit which effectively corresponds to the illustration in FIG. 1 of the U.S. Patent of Kaiser et al, U.S. Pat. No. 4,064,530 entitled "Noise Reduction System for Color Television". Video signals applied to an input terminal 41 are coupled through a variable attenuator 43 which scales the video input signal values by the difference between unity and a transmission constant `a`, the value of which is some fraction of unity. The output of variable attenuator 43 is coupled to one input of adder 45 a second input of which is coupled to the output of a variable attenuator 53 whose transmission constant is `a`. The output of adder 45 is coupled to a frame delay 47, the output of which is subject to chroma inverter 51 and applied to output terminal 55. The output of chroma inverter 55 is itself scaled by attenuator 53 and applied to adder 45 to be combined with the fractionally scaled video input signal.